The present invention relates to an electrically charged filter and a mask provided with an electrically charged filter.
Conventionally, in order to capture dust in the air, a filter made of nonwoven fabric has been used.
A filter of this kind made of nonwoven fabric has been required to have as high a filtering efficiency as possible. The capture of dust in a filter made of nonwoven fabric is principally by means of Brownian diffusion, interception, inertial impaction or the like, based on a physical action. Accordingly, if the diameter of the fibers making up the filter is reduced, smaller dust particles and the like can be captured and removed, and therefore it is possible to increase the dust filtering efficiency.
However, the smaller the diameter of the fibers is made in order to improve the filtering efficiency, the greater the initial pressure loss and hence there have been problems in that air permeability is poor. Furthermore, it is known that as the capture of dust by a filter made of nonwoven fabric progresses, clogging occur due to dust being held in the spaces in the filter, and the pressure loss progressively increases.
Therefore, a filter has been sought which has low initial pressure loss, despite having high filtering efficiency, and which suppresses increase in the pressure loss due to the capture of dust.
As a method for resolving this problem, attempts have been made to improve filtering efficiency, reduce initial pressure loss and suppress increase in pressure loss, by electrically charging a filter made of nonwoven fabric and thus using dust capture based on an electrostatic capturing action in addition to dust capture based on a physical capturing action.
For an electrically charged filter of this kind, an electret body (Patent Literature 1) is known which is obtained by applying an ultrasonic wave to a structure made from thermoplastic resin via a polar liquid. The electret body according to Practical Example 1 disclosed in Patent Literature 1 is obtained by applying an ultrasonic wave to a nonwoven fabric prepared by melt-blowing technology via a polar liquid, and has excellent filtering efficiency of airborne dust particles.